In an equally bad English, I will say that IRI is no form from ESSE at all. It is the present infinitive passive of IRE "to go". It is used to build the future infinitive passive, e. g. CAPTUM IRI "to be later caught". When the Latin says CREDO URBEM CAPTUM IRI "I believe that the town will be caught", he thinks "I believe that one is going to catch te town". IRI is an impersonal passive ("one goes") and CAPTUM is a supine denoting purpose after a verb of motion. URBEM here is no subject but a direct object of CAPTUM.

Salve,
Strange that you would think iri was from esse, a friend of mine was recently going on about how it was the present passive infinitive of esse. Perhaps because other infinitives (future active, perfect passive) use esse?